The present invention relates to an apparatus and method for automatically adjusting luminance and contrast of a display unit. Specifically, the present invention is directed to an apparatus and method that adjust luminance and contrast as functions of ambient and surround luminance.
There are a variety of applications and devices in which a display unit is utilized to electronically display images. It is desirable to provide a display unit for such applications and devices that is capable of automatically adjusting luminance and contrast without the need for operator intervention. It is common, for example, to provide electronic cameras with image display units that must be utilized in a wide range of lighting conditions in which luminance levels vary greatly. Without some form of automatic adjustment, the operator must constantly change settings manually to compensate for changes in the lighting environment.
It has been well documented in the scientific literature that one""s brightness and contrast perception of an image is significantly affected by the ambient and surround luminance in which it is viewed. See, for example, xe2x80x9cVisibility of Transmissive Liquid Crystal Displays under Dynamic Lighting Conditionsxe2x80x9d, J. H. Krantz, Human Factors, 1992, 34(5), 615-632, the contents of which are herein incorporated by reference. Brightness is defined as the attribute of a visual sensation according to which an area appears to emit more or less light. Contrast is defined as the rate of change of the brightness of image elements as a function of the relative luminance of the same image elements in the original scene. Differences in these perceived attributes can be related to the operator""s state of visual adaptation to the ambient and surround luminance. For example, as the ambient luminance increases, the user begins to gradually adapt to the new light level by: a) reducing the amount of light which enters the eye; and b) reducing the sensitivity of the light-sensitive elements of the eye. The light entering the eye from the display unit, however, is also correspondingly reduced, thereby causing the display unit to appear dimmer to the operator. In response to this effect, the luminance of the display unit needs to be increased to maintain the operator""s perception of the displayed image. Conversely, as the ambient luminance decreases, the operator begins to gradually adapt by: a) increasing the amount of light that enters the eye; and b) by increasing the sensitivity of the light-sensitive elements of the eye. In this case, the light entering the eye from the display device unit is also increased, thereby causing the display unit to appear brighter to the operator. The luminance of the display unit must then be decreased to maintain the user""s perception of the displayed image.
While the eye adapts to changes in ambient luminance, the process of adaptation is not directly reversible. Instead, the human eye requires a significant period of time to adapt to changes when ambient luminance is decreased and a much shorter period to adapt to increases in ambient luminance. FIG. 1, for example, illustrates a plot of the darkness adaptation effect showing the amount of time it takes for an individual to visually adapt to changes in ambient luminance. As illustrated in FIG. 1, several minutes are required before the sensitivity of the eye reaches its full level of adaptation when the ambient luminance is decreased. FIG. 2 is a plot of the lightness adaptation effect showing the amount of time it takes for the eye to adapt to increased ambient luminance. As illustrated in FIG. 2, the eye""s sensitivity changes dramatically over the first second after an increase in ambient luminance, continues to change significantly for a period of 20 to 30 seconds, and then changes very slightly thereafter.
It has also been shown that the perceived contrast of an image changes as the luminance of the image surround changes. The surround of an image is defined as the area around an image extending from its edge in all directions. FIG. 3 is a plot which shows such an effect. The plot illustrates the relative brightness of image elements as a function of their relative luminances for an average, dim, and darkly illuminated surround, which shows that the perceived image contrast increases with increasing surround luminance and decreases with decreasing surround luminance.
It is also known that the physical luminance and contrast of the display unit will be affected by lighting conditions. Assuming that the display unit reflects light as if it were a perfectly Lambertian surface, it is known that the luminance of the display unit in any ambient environment can be analytically determined from the equation:
La=Ld+Lr
where La represents the luminance of the display unit in current ambient lighting conditions, Ld represents the luminance of the display unit in a perfectly dark environment, and Lr represents the reflected luminance of the display unit. If the amount of ambient illuminance is measured, Lr can be calculated from the equation:
Lr=I*R/xcfx80
where I is the ambient illuminance, R is the coefficient of reflection for the display device and xcfx80 is the arithmetic constant PI. The contrast of the display device can be determined by calculating La for a dark pixel and La for a fully lit pixel on the display device and then calculating the ratio of these parameters.
Contrast of Display=Lafully lit pixel/Ladark pixel
It should be noted that while R will usually be constant for most display units, this parameter may differ for dark and lit pixels for some display units such as liquid crystal displays.
Still further, researchers have found that the optimum display luminance is a function of display illumination level and this relationship can generally be explained by the equation:
xe2x80x83log Ld=a+b log(I)
where Ld is the display luminance, I is the illumination level, and a and b are constants fit to psychophysical data. See xe2x80x9cThe ABC""s of Automatic Brightness Controlxe2x80x9d, R. Merrifield and L. D. Silverstein, SID 88 Digest, 1988, pgs. 178-180, the contents of which are incorporated herein by reference.
In view of the above, it is an object of the invention to provide an apparatus and method of dynamically modifying both the luminance and contrast of an image as it is displayed on a display unit in response to changing lighting conditions.
The invention provides an apparatus and method for dynamically modifying both the luminance and contrast of an image as it is displayed on a display unit in response to changing lighting conditions. Sensors are utilized to continually measure the luminance of the light illuminating the display unit and/or the display surround luminance. Measurement signals generated by the light sensors are processed to provide display luminance and contrast adjustment control signals that gradually cause the adjustment of the display unit""s luminance and contrast in response thereto.
More specifically, an apparatus for automatically controlling a display luminance and contrast of a display device is provided that includes an illumination measuring mechanism for measuring a display illumination of a display device; a surround luminance measuring mechanism for measuring a surround luminance of the display device; a calculating mechanism for calculating a current display luminance and a current contrast setting for the display device; a determining mechanism for determining a change in the display illumination and a change in the surround luminance based on measurements performed by the illumination measuring mechanism and the surround luminance measuring mechanism, respectively, a luminance adjustment mechanism for calculating an updated display luminance when the determining mechanism determines a change in the display illumination has occurred, and for adjusting the display luminance of the display device based on the updated display luminance to display an image, and a contrast adjustment mechanism for calculating an updated contrast setting when the determining mechanism determines a change in the surround luminance has occurred, and for adjusting the contrast of the display device based on the updated contrast setting to display the image.
In a preferred embodiment, the determining mechanism calculates the current display luminance based on a determination of whether an initial display illumination measured by the illumination measuring mechanism is different from a default display illumination stored in memory. Similarly, the determining mechanism preferably calculates the current contrast setting based on a determination of whether an initial surround luminance measured by the surround luminance measuring mechanism is different from a default surround luminance stored in memory.
Still further, the luminance adjustment mechanism calculates the updated display luminance based on the change in display illumination, and further performs a progressive time dependent adjustment of the display luminance based on whether the updated display luminance is higher or lower than the current display luminance. The luminance adjustment mechanism performs the progressive time dependent adjustment based on a degree of light adaptation when the updated luminance is higher than the current display luminance, and performs the progressive time dependent adjustment based on a degree of dark adaptation when the updated luminance is lower than the current display luminance.
The present invention provides continual adjustment of the luminance and contrast of a display unit according to changing lighting conditions such that the brightness and contrast perception of the displayed image remains constant under the varying conditions. As a result, a number of advantages are obtained including: the brightness and contrast perception of images displayed on a display unit remains constant as the display device illumination changes; the brightness and contrast perception of images displayed on a display unit remains constant as the display unit surround luminance changes; and power consumption is decreased in dim environments. Other advantages and features of the invention will become apparent from the following detailed description of the preferred embodiments of the invention.